National Weather Service Weather Forecast Office

Area Forecast Discussion

269
FXUS64 KHUN 191634
AFDHUN

AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE HUNTSVILLE AL
1134 AM CDT SUN MAY 19 2013

.UPDATE...
LOWERED/ADJUSTED POPS...TWEAKED SKY COVER AND TEMPS.

&&

.DISCUSSION...
UPPER RIDGE AXIS IS NOW OVER THE MS/AL STATE LINE WHICH PUTS THE ERN
HALF OF OUR CWA IN NW FLOW. THERE WERE A FEW UPPER DISTURBANCES
CAUGHT UP IN THIS NW FLOW WHICH WAS DUMPING VERY HVY RAIN OVER
PORTIONS OF ERN TN AND THE NRN HALF OF GA. AS THIS RIDGE BUILDS EAST
THIS AFTN...THE CHC OF PCPN ACROSS THE TN VALLEY WILL TAPER OFF. FOR
NOW WILL GO WITH A LOW POP (20/30 PERCENT) FOR LOCATIONS ALONG AND
EAST OF I65...WEST OF I65 WILL END THE CHC OF PCPN. NO SVR WX IS
EXPECTED THIS AFTN...HOWEVER A FEW STORMS MAY PRODUCE WIND GUSTS UP
TO 30 MPH ALONG WITH SMALL HAIL AND BRIEF HVY RAIN.

07

&&

.AVIATION...
/ISSUED 629 AM CDT SUN MAY 19 2013/
FOR 12Z TAFS...
SHALLOW FOG HAS RESULTED IN MVFR TO MARGINAL IFR VSBY AT BOTH
TERMINALS EARLY THIS MRNG. SATL IMAGERY SHOWS LEADING EDGE OF LOWER
CIGS (~600 FT AGL) ADVECTING NORTHWEST ACROSS THE AREA. EXPECT THESE
LOWER /IFR/ CIGS TO IMPACT KHSV AT LEAST THROUGH 14Z. OTHERWISE...VFR
CONDITIONS WILL GRADUALLY PREVAIL AND INCREASING WIND/MIXING ERODE
HAZE/FOG LAYER WITHIN MOIST BNDRY LAYER. ANY SHRA/TSRA SHOULD STAY
EAST OF THE TERMINALS TODAY...HOWEVER CANNOT RULE OUT A STRAY
-SHRA/-TSRA. FORECASTERS WILL NEED TO MONITOR THIS POTENTIAL THROUGH
THE MORNING HOURS.

DJN.83

&&

.PREV DISCUSSION...
/ISSUED 452 AM CDT SUN MAY 19 2013/
A SEASONABLY WARM/MOIST AIRMASS IS IN PLACE ACROSS NORTHERN
ALABAMA/SOUTHERN MIDDLE TENNESSEE THIS MORNING...WITH ELEVATED WARM
ADVECTION ABOVE SEVERAL LOW-LEVEL BOUNDARIES RESULTING IN ISOLATED TO
WIDELY SCATTERED SHOWERS AND THUNDERSTORMS...WITH THIS ACTIVITY
LIKELY TO EXPAND/INTENSIFY AS DESTABILIZATION OCCURS THIS MORNING.
MEANWHILE...A MID-LEVEL SHORTWAVE TROUGH -- CURRENTLY LOCATED ACROSS
CENTRAL KENTUCKY/MIDDLE TENNESSEE -- IS FORECAST TO DIG SLOWLY
SOUTHEASTWARD INTO THE SOUTHERN APPALACHIANS TODAY. THUS...AS WE GO
THROUGH THE DAY...THE SENSIBLE WEATHER ACROSS THE CENTRAL TENNESSEE
VALLEY WILL BE LESS INFLUENCED BY THE TROUGH...AND INCREASINGLY
INFLUENCED BY A HIGH AMPLITUDE MID-LEVEL RIDGE ALIGNED ACROSS THE
MISSISSIPPI RIVER VALLEY THIS MORNING. THIS RIDGE IS FORECAST TO
MIGRATE EASTWARD ACROSS THE FORECAST AREA TODAY...AS A CLOSED LOW
EVOLVES ACROSS THE CENTRAL HIGH PLAINS -- WITHIN A BROADER LONGWAVE
TROUGH ENCOMPASSING MUCH OF THE WESTERN CONUS. MID-LEVEL WARMING AND
HEIGHT RISES ASSOCIATED WITH THE INCREASINGLY SUBSIDENT AIRMASS ALOFT
SHOULD RESULT IN A FAIRLY SHARP POP GRADIENT ACROSS THE REGION THIS
MORNING...WITH THE GREATEST PROBABILITIES FOR SHOWERS/STORMS
GENERALLY EAST OF INTERSTATE 65. PRECIPITATION CHANCES WILL DECREASE
SUBSTANTIALLY FOR ALL AREAS BY THIS AFTERNOON...BUT SLIGHTLY HIGHER
POPS WILL BE MAINTAINED ACROSS THE NORTHEASTERN PORTION OF THE
FORECAST AREA. DESPITE THE PRESENCE OF HIGH-LEVEL DEBRIS CLOUDS FROM
CONVECTION ACROSS THE CENTRAL PLAINS...LOW-LEVEL CLOUDS SHOULD
DISSIPATE FROM WEST-TO-EAST THROUGH THE AFTERNOON ALLOWING
TEMPERATURES TO WARM INTO THE LOWER/MID 80S WITH A SOUTH-SOUTHWEST
WIND OF 5-10 MPH.

GLOBAL MODELS ARE IN GOOD AGREEMENT THAT A STRONG BLOCKING
ANTICYCLONE POSITIONED ACROSS CENTRAL MANITOBA/SASKATCHEWAN WILL
FORCE THE CENTRAL HIGH PLAINS CLOSED LOW TO DRIFT VERY SLOWLY
NORTHEASTWARD FROM TONIGHT THROUGH THE END OF THE SHORT TERM
PERIOD...LIKELY REACHING WESTERN MINNESOTA BY TUESDAY. WEATHER
CONDITIONS ACROSS THE LOCAL AREA WILL CONTINUE TO BE PRIMARILY
INFLUENCED BY A STRONG DOWNSTREAM MID-LEVEL RIDGE WHICH SHOULD BECOME
POSITIONED NEAR OR JUST SOUTHEAST OF THE AREA. ALTHOUGH A FEW
AFTERNOON SHOWERS/THUNDERSTORMS MAY SPREAD SOUTHWESTWARD INTO THE
EASTERN COUNTIES FROM THE SOUTHERN APPALACHIANS ON TUESDAY
AFTERNOON...EXPECT GENERALLY DRY WEATHER TO PREVAIL AND WILL ONLY
CARRY A SLIGHT CHANCE POP ON TUESDAY. VERY WARM AND HUMID CONDITIONS
CAN ALSO BE EXPECTED...WITH A 15-25 KNOT SOUTHWESTERLY LOW-LEVEL JET
PERHAPS LEADING TO SOME MORNING STRATUS CLOUDS BOTH ON MONDAY AND
TUESDAY...ESPECIALLY ACROSS NORTHWEST ALABAMA. AFTERNOON HIGHS WILL
REACH INTO THE UPPER 80S/LOWER 90S FOR MOST OF THE AREA -- WITH THE
EXCEPTION OF THE HIGHER TERRAIN OF NORTHEAST ALABAMA/SOUTHERN MIDDLE
TENNESSEE. THIS...COUPLED WITH DEWPOINTS IN THE MID 60S-70 DEGREE
RANGE WILL YIELD AFTERNOON HEAT INDICES IN THE LOWER 90S.

IN THE EXTENDED FORECAST...IT APPEARS THAT THE OVERALL PATTERN WILL
BEGIN TO CHANGE ON TUESDAY NIGHT/WEDNESDAY...AS THE MID-LEVEL HIGH
ACROSS SOUTHERN CANADA BUILDS NORTHWARD IN RESPONSE TO A STRONG MID-
LATITUDE TROUGH DIGGING SOUTHEASTWARD INTO THE PACIFIC NORTHWEST
REGION. AS THIS OCCURS...THE NORTHERN PLAINS CYCLONE WILL LIKELY
BECOME ABSORBED WITHIN THE STRONGER CIRCULATION AROUND AN UPPER LOW
DROPPING SOUTHWARD ACROSS THE EASTERN EDGE OF HUDSON BAY. ALTHOUGH A
FEW SHOWERS/STORMS WILL BE POSSIBLE ACROSS NORTHWEST ALABAMA EARLY
WEDNESDAY MORNING...EXPECT PRECIPITATION CHANCES TO INCREASE MOST
SUBSTANTIALLY DURING THE DAY ON WEDNESDAY AS A TRAILING MID-LEVEL
SHEAR AXIS AND ASSOCIATED/WEAK SURFACE TROUGH APPROACH FROM THE WEST.
ELEVATED CHANCES FOR SHOWERS AND THUNDERSTORMS WILL PERSIST INTO
WEDNESDAY NIGHT...AS THE PREVIOUSLY MENTIONED FEATURES CONTINUE TO
ADVANCE EASTWARD ACROSS THE REGION -- MAINTAINING STRONG SYNOPTIC
SCALE LIFT IN THE WARM/MOIST AIRMASS IN PLACE ACROSS THE SOUTHEASTERN
U.S. DURING THE PERIOD FROM THURSDAY THROUGH FRIDAY...GUIDANCE FROM
THE OPERATIONAL GFS/ECMWF IS IN DISAGREEMENT ON HOW QUICKLY A
SLIGHTLY COOLER/DRIER CP AIRMASS WILL SPREAD INTO THE REGION...WITH
THE GFS THE SLOWER OF THE TWO SOLUTIONS -- BRINGING THE DRIER AIR
INTO THE REGION ON FRIDAY NIGHT...AND THUS MAINTAINING SCATTERED
SHOWERS/STORMS ON THURSDAY AND FRIDAY. THE ECMWF IS MORE PROGRESSIVE
AND BRINGS THE DRIER AIRMASS INTO THE AREA LATE THURSDAY.
REGARDLESS...IT APPEARS THAT A DRIER AND SLIGHTLY COOLER AIRMASS
WILL BE IN PLACE AT THE BEGINNING OF NEXT WEEKEND.

70/DD

&&

.HUN WATCHES/WARNINGS/ADVISORIES...
AL...NONE.
TN...NONE.
&&

$$

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